Fine wire made of a gold alloy, method for its production, and its use

ABSTRACT

A fine wire made of an alloy of gold which contains 0.6 to 2 weight % of nickel, or an alloy of gold which contains 0.1 to 2 weight % of nickel, 0.0001 to 0.1 weight % of alkaline earth metal and/or rare earth metal, and optionally 0.1 to 1.0 weight % of platinum and/or palladium . The fine wire is distinguished by a favorable electrical conductivity and a good ratio of strength to elongation. The fine wire is suitable both for wire bonding of semiconductor devices and for producing the ball bumps of flip-chips.

BACKGROUND INFORMATION

1. Field of the Invention

The present invention relates to a fine wire of a gold alloy containingnickel for bonding semiconductor devices, to a method for itsproduction, and to its use.

2. Background of the Invention

For contacting or bonding semiconductor devices, suitable wires, alsoknown as bonding wires, must have good electrical properties and goodmechanical strength values. The diameter of the wires can beapproximately 10 to 200 μm and is typically in the range ofapproximately 20 to 60 μm; the diameter is selected to suit the intendedpurpose.

The bonding wires often comprise gold of high purity, or recently alsogold alloys. The latter have the advantage of greater strength and, ifthey contain only a slight quantity of alloy formers, their electricalconductivity is reduced only slightly over pure gold.

For instance, the use of an alloy of gold and 0.001 to 0.1% of one ormore rare earth metals, especially in the form of cerium mixed metal, oryttrium is known from German Patent DE 16 08 161 C for producing leadwires in integrated circuits. This alloying of the gold with slightquantities of rare earth metals or yttrium has markedly better strengthand elongation performance at heating temperatures up to 500° C.,without any substantial change in other properties of the gold, such ashardness, chemical resistance, or electrical resistance.

Gold and rare earth metal alloys for bonding wires are also described inGerman Patent Disclosures DE 32 37 385 A (U.S. Pat. No. 4,885,135) andDE 39 36 281 A (U.S. Pat. No. 4,938,923), Japanese Patent Disclosure JP6-112258 A, and European Patent Disclosures EP 0 743 679 A and EP 0 761831 A.

DE 32 37 385 A relates to a fine gold alloy wire with high tensilestrength, comprising a gold alloy with 0.0003 to 0.01 weight % of rareearth metal, especially cerium, and optionally germanium, berylliumand/or calcium in addition.

DE 39 36 281 A describes a gold wire for connecting a semiconductordevice, comprising high-purity gold alloyed with slight quantities oflanthanum, beryllium, calcium, and elements from the platinum group,especially platinum and/or palladium.

The bonding wire known from JP 6-112258 A, reported in ChemicalAbstracts, Vol. 121, 89287m, comprises a gold alloy with from 1 to 30%of platinum and 0.0001 to 0.05% of scandium, yttrium and/or rare earthmetal, and optionally 0.0001 to 0.05% of beryllium, calcium, germanium,nickel, iron, cobalt, and/or silver.

In EP 0 743 679 A, a bonding wire of a gold and rare earth metal alloycontaining platinum is also proposed. The alloy comprises gold andslight quantities of platinum (0.0001 to 0.005 weight %), silver,magnesium and europium, and can also contained cerium, for example, in aquantity of from 0.0001 to 0.02 weight %.

In EP 0 761 831 A, a fine wire comprising a platinum-and/orpalladium-containing gold and rare earth metal alloy is described. Thealloy comprises 0.1 to 2.2 weight % of platinum and/or palladium, 0.0001to 0.005 weight % of beryllium, germanium, calcium, lanthanum, yttriumand/or europium, the remainder being gold. The wire is produced bymelting the elements forming the alloy in a crucible, with cooling frombottom to top of the alloy melt in the crucible to obtain a casting(ingot), and ensuing rolling, drawing and annealing. It has anelongation of 3 to 8% and a Young modulus of 6800 to 9000 kgf/mm².

From JP 52-051867 A, bonding wires of gold and 0.004 to 0.5 weight % ofat least one of the metals in the group comprising nickel, iron, cobalt,chromium and silver is known. The bonding wires, with a diameter of 30μm, have good bonding properties and—compared with bonding wires of puregold—improved strength. Nickel in a quantity of 0.004 weight %, at 6%elongation produces a strength of 13 kg/mm² (130 N/mm²), and in aquantity of 0.5 weight % at 14% elongation produces a strength of 24kg/mm² (240 N/mm²). Higher quantities of nickel are thought to reducethe mechanical properties; for a bonding wire of a gold alloy with 0.6weight % of nickel at an elongation of 5%, a strength of then only 10kg/mm² (100 N/mm²) is for instance stated. The production of the bondingwires is not described in JP 52-051867 A.

In East German Patent Disclosure DD 201 156, gold/silver alloys forbondable microwires are proposed that as additives contain copper,nickel and/or cobalt in concentrations of ≦5 mass % and iron, aluminum,palladium, platinum, antimony, bismuth, germanium, and arsenic astypical contaminants (not above 100 ppm). The alloys are melted in thevacuum induction furnace and cast to ingots. The subsequent extrusion ofthe ingots is followed by cold forming to the final diameter (25 to 30μm) with suitable heat treatments.

Gold/nickel alloys are also known for other purposes. For instance,Published, Examined German Patent Application 1 169 140 teaches the useof a gold/nickel alloy with 1 to 20 weight % of nickel as material forproducing weak-current contacts for circuits with self-induction in therange from 10⁻⁷ to 10⁻⁴ Henry. To raise the temperature ofrecrystallization, the gold/nickel alloy can also contain silver,platinum, palladium, zirconium, copper, cobalt, iron, chromium and/ormanganese.

SUMMARY OF THE INVENTION

With JP 52-051867 A as its point of departure, an object of theinvention was to discover a fine wire of a gold alloy that containsnickel, and which has a good ratio of strength to elongation. A methodis also disclosed which in an economical way enables continuousproduction of the fine wire with excellent quality. The fine wire shouldbe suitable both for wire bonding and for producing ball bumps for theflip-chip technique, as described, for instance, in German Patent DE 4442 960 C.

According to the present invention, the object is attained by a finewire comprising an alloy, which is characterized in that it comprisesgold and from 0.6 to 2 weight % of nickel.

According to the present invention, the object is also attained by afine wire comprising an alloy, which is characterized in that itcomprises from 0.1 to 2 weight % of nickel and 0.0001 to 0.1 weight % ofat least one element selected from the group comprising the alkalineearth metals and rare earth metals, with the remainder being gold.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a graph of tensile strength versus elongation at fracturefor Examples 1 to 4 hereinbelow.

DETAILED DESCRIPTION OF THE INVENTION

The fine wire of the present invention has proved to be especiallyuseful if the nickel content of the gold alloy is from 0.7 to 1.5 weight%. Preferably, the alkaline earth metal and/or rare earth metal contentof the gold alloy is from 0.001 to 0.01 weight %, if present.

In some cases, fine wires of a gold/nickel alloy that additionallycontains 0.1 to 1.0 weight % of platinum, palladium, or both platinumand palladium have proved to be highly advantageous.

In terms of the present invention, the term “alkaline earth metal” isunderstood to mean beryllium, magnesium, calcium, barium and strontium,while “rare earth metal” means lanthanum (ordinal number 57) and the 14elements following lanthanum, that is, cerium (ordinal number 58)through lutetium (ordinal number 71), also known in the professionalliterature as “elements of the lanthanum series”.

The alkaline earth metal preferably comprises beryllium, magnesium,calcium or a mixture of at least two of these alkaline earth metals. Ifmixtures of beryllium and calcium are used, those comprising 50 weight %each of beryllium and calcium have proved to be especially suitable.

The rare earth metal preferably comprises cerium, or a mixture of ceriumand one or more of the rare earth metals having the ordinal numbers 57and 59-71. Cerium mixed metal has proved to be especially suitable.Typically a mixture of 50-60% cerium, 25 to 30% lanthanum, 10 to 15%neodymium, 4 to 6% praseodymium and 1% iron, and slight amounts of otherrare earth metals is usually called a cerium mixed metal (Römpp ChemieLexikon (Römpp's Chemical Lexicon), Georg Thieme Verlag Stuttgart—NewYork, Vol. 1, 10th Edition (1996), 647).

The fine wire according to the present invention with a typical diameterfor bonding wires has all the properties necessary for its use forsemiconductor bonding. It is distinguished especially by its favorableelectrical conductivity (see Table V hereinbelow), measured in the formof specific electrical resistance, and its very good strength inrelation to the elongation (see the Figure). The good strength toelongation ratio of the fine wire contributes substantially to the veryquality of the bonded connections.

Surprisingly, the alloy forming of the gold with nickel or with nickeland alkaline earth metal and/or rare earth metal in the quantitiescomplained leads to the higher strength of the fine wire according tothe invention, compared with the wires of unalloyed gold and ofgold/nickel alloys of JP 52-051867 A. It is also especially surprisingthat by also alloying alkaline earth metal and/or rare earth metal, agreat reduction in the loss of strength from annealing (see Table VIhereinbelow) can be attained.

In the drawing (figure), the strength (tensile strength) [N/mm²] of twofine wires according to the present invention (in Examples 1 and 2) and,for comparison, two fine wires not according to the invention (Examples3 and 4) is shown as a function of the elongation (breaking elongation)(%): The fine wires of the present invention have greater strength for agiven elongation.

In Table V, the chemical composition and the specific electricalresistance of the fine wires both according to the present invention andnot according to the embodiments described in the examples and inaddition of a fine wire of a gold alloy with 0.8 weight % of iron areshown. Table VI shows the values for the strength of the fine wiresdescribed in the examples in the cold-drawn state and at an elongationof 4% and shows the influence of adding beryllium and calcium on thestrength. Beryllium and calcium reduce the loss of strength associatedwith the annealing. The fine wire according to the present invention,because of its favorable properties, can especially advantageously beused for wire bonding including the high-frequency bonding techniquebeing developed, and for producing the ball bumps for flip-chips.

The object of the present invention is also attained in a method forproducing a fine wire from a nickel-containing gold alloy for bondingsemiconductor components, which according to the present invention ischaracterized in that a gold alloy comprising (a) gold, 0.6 to 2 weight% of nickel, and optionally 0.1 to 1.0 weight % of platinum, palladiumor both platinum and palladium, or (b) gold, 0.1 to 2 weight % ofnickel, 0.0001 to 0.1 weight % of at least one element selected from thegroup of alkaline earth metals and rare earth metals, and optionally 0.1to 1.0 weight % of platinum, palladium, or both platinum and palladium,with the remainder being gold, is melted; the molten alloy iscontinuously cast into a strand; the strand is drawn into a wire oftypical diameter for bonding purposes; and the wire is annealed.

The method according to the present invention has proved to beespecially useful if the molten alloy is continuously cast into a strandof circular cross section, the strand is drawn into a wire, and the wireis annealed at about 300 to 700° C. By the annealing, the initiallycold-drawn wire is imparted with the necessary elongation. The meltingand continuous casting of the alloy can be done in air, in a protectivegas such as argon, or in a vacuum.

In the method of the present invention, melting of a gold alloy with anickel content of 0.7 to 1.5 weight % is preferred. The addition ofalkaline earth metal and/or rare earth metal in a quantity of 0.001 to0.01 weight % has proven favorable.

As the alkaline earth metal, beryllium, magnesium, calcium, strontium,barium, or a mixture of at least two of these elements can be used.Beryllium, magnesium, calcium or a mixture of at least two of thesealkaline earth metals has proved especially suitable. If mixtures ofberyllium and calcium are used, then those comprising 50 weight % eachof beryllium and calcium are preferred.

As the rare earth metal, especially cerium or a mixture of cerium andone or more rare earth metals with the ordinal numbers 57 and 59-71 isused, the latter preferably in the form of commercially available ceriummixed metal.

The method of the present invention is especially distinguished in thatit can be performed continuously and yields products, the cast strandand the drawn wire, of very uniform and constant quality.

For further explanation, in the following examples, fine wires and theirproduction in accordance with the present invention (Examples 1 and 2)and, for comparison, a fine wire of the prior art known from DE 16 08161 C (Example 3) and a fine wire of gold with a purity of 99.99 weight% (Example 4) will be described. The fine wires are characterized bytheir elongation (breaking elongation) (%), their strength (tensilestrength) (N/mm²), and their specific electrical resistance (ohmsmm²/m).

EXAMPLES Example 1

Fine wire comprising a gold alloy with 0.8 weight % of nickel

The melt of an alloy of 0.8 weight % of nickel and gold as the remainderis cast in a continuous-casting system to a strand of circular crosssection. From the strand, a wire with a diameter of 30 μm is then drawn,and the wire is annealed in air at approximately 300 to 700° C.depending on the elongation to be attained. The strength values (N/mm²)measured as a function of the elongation (%) are shown in Table I.

The specific electrical resistance at room temperature, measured for awire with a diameter of 275 μm, is 0.045 ohms mm²/m.

TABLE I Elongation (%) Strength (N/mm²) cold-drawn 600 2.7 357 2.8 3293.4 310 5.0 285 6.4 264 7.6 254

Example 2

Fine wire comprising a gold alloy with 0.8 weight % of nickel, 0.001weight % of beryllium and 0.001 weight % of calcium

The melt of an alloy of 0.8 weight % of nickel, 0.001 weight % ofberyllium and 0.001 weight % of calcium, with gold as the remainder, iscast in a continuous-casting system to a strand of circular crosssection. From the strand, a wire with a diameter of 30 μm is then drawn,and the wire is annealed in air at approximately 300 to 700° C.depending on the elongation to be attained. The strength values (N/mm²)measured as a function of the elongation (%) are shown in Table II.

The specific electrical resistance at room temperature, measured for awire with a diameter of 275 μm, is 0.046 ohms mm²/m.

TABLE II Elongation (%) Strength (N/mm²) cold-drawn 650 3.8 433 4.0 4524.5 405 4.8 380 5.3 354 5.8 333 6.7 309 8.8 284

Example 3 (comparison)

Fine wire of a gold alloy with cerium mixed metal in accordance with DE16 08 161 C

The melt of an alloy of gold and cerium mixed metal is cast in acontinuous-casting system to a strand of a circular cross section. Fromthe strand, a wire with a diameter of 30 μm is then drawn, and the wireis annealed in air at approximately 300 to 600° C. depending on theelongation to be attained. The strength values (N/mm²) measured as afunction of the elongation (%) are shown in Table III.

The specific electrical resistance at room temperature, measured for awire with a diameter of 275 μm, is 0.023 ohms mm²/m.

TABLE III Elongation (%) Strength (N/mm²) cold-drawn 375 2.9 263 3.1 2533.6 243 4.0 230 5.7 220 8.1 209 10.1  198

Example 4 (comparison)

Fine wire of gold with a purity of 99.99 weight %

The melt of gold with a purity of 99.99 weight % is cast in acontinuous-casting system to a strand of a circular cross section. Fromthe strand, a wire with a diameter of 30 μm is then drawn, and the wireis annealed in air at approximately 200 to 500° C. depending on theelongation to be attained. The strength values (N/mm²) measured as afunction of the elongation (%) are shown in Table IV.

The specific electrical resistance at room temperature, measured for awire with a diameter of 275 μm, is 0.023 ohms mm²/m.

TABLE IV Elongation (%) Strength (N/mm²) cold-drawn 435 2.1 235 2.2 2312.5 226 3.0 221 3.6 214 3.8 192 4.6 182 5.8 171

TABLE V Specific electrical Composition (weight %) resistance Example AuBe Ca Ni Fe (ohms mm²/m) 1 remainder 0.8 0.045 2 remainder 0.001 0.0010.8 0.046 3 (comparison)* 0.023 4 (comparison) 99.99 0.023 (comparison)remainder 0.8 0.227 *Gold alloy with cerium mixed metal in accordancewith DE 16 08 161 C

TABLE VI Tensile strength Composition (N/mm²) (weight %) Drawn- 4%Example Au Be Ca Ni hard elongation 1 remainder 0.8 600 300 2 remainder0.001 0.001 0.8 650 450 3 (comparison)* 375 230 4 (comparison) 99.99 435200 *Gold alloy with cerium mixed metal in accordance with DE 16 08 161C

It will be appreciated that the instant specification is set forth byway of illustration and not limitation, and that various modificationsand changes may be made without departing from the spirit and scope ofthe present invention.

What is claimed is:
 1. A fine wire of a gold alloy for bonding semiconductor devices, wherein the gold alloy consists essentially of 0.6 to 2 weight % of nickel, 0.1 to 1.0 weight % of at least one metal selected from the group consisting of platinum and palladium, with the remainder being gold.
 2. The fine wire in accordance with claim 1, wherein the nickel is in an amount of 0.7 to 1.5 weight %.
 3. The fine wire in accordance with claim 1, wherein the wire has a diameter of 10 to 200 μm.
 4. The fine wire in accordance with claim 1, wherein the wire has a diameter of 20 to 60 μm.
 5. A fine wire of a gold alloy for bonding semiconductor devices, wherein the gold alloy consists essentially of 0.1 to 2 weight % of nickel, 0.0001 to 0.1 weight % of at least one element selected from the group consisting of an alkaline earth metal and a rare earth metal, and 0.1 to 1.0 weight % of at least one metal selected from the group consisting of platinum and palladium, with the remainder being gold.
 6. The fine wire in accordance with claim 5, wherein the nickel is in an amount of 0.7 to 1.5 weight %.
 7. The fine wire in accordance with claim 5, wherein the at least one element selected from the group consisting of the alkaline earth metal and the rare earth metal is in an amount of 0.001 to 0.01 weight %.
 8. The fine wire in accordance with claim 6, wherein the gold alloy contains at least one element selected from the group consisting of the alkaline earth metal and the rare earth metal in an amount of 0.001 to 0.01 weight %.
 9. The fine wire in accordance with claim 5 wherein the alkaline earth metal which is contained in the gold alloy is at least one element selected from the group consisting of beryllium, magnesium and calcium.
 10. The fine wire in accordance with claim 5, wherein the rare earth metal which is contained in the gold alloy is cerium.
 11. A method for producing a fine wire according to claim 1 from a gold alloy containing nickel, for bonding semiconductor devices comprising: (a) melting a gold alloy to provide a molten alloy, said gold alloy selected from the group consisting of (i) a gold alloy consisting essentially of 0.6 to 2 weight % nickel, 0.1 to 1.0 weight % of at least one metal selected from the group consisting of platinum and palladium, with the remainder being gold and (ii) a gold alloy consisting essentially of 0.1 to 2 weight % nickel, 0.0001 to 0.1 weight % of at least one element selected from the group consisting of an alkaline earth metal and a rare earth metal, and 0.1 to 1.0 weight % of at least one metal selected from the group consisting of platinum and palladium, with the remainder being gold; (b) continuously casting the molten alloy from step (a) into a strand; (c) drawing the strand from step (b) into a wire of a diameter for bonding to a superconductor device; and (d) annealing the wire from step (c).
 12. The method in accordance with claim 11, wherein in step (b) the molten alloy is cast into a strand of a circular cross section.
 13. The method in accordance with claim 11, wherein in step (d) the wire is annealed at a temperature of 300 to 700° C.
 14. A method of bonding a semiconductor device with a wire, comprising bonding a semiconductor device with the fine wire according to claim
 1. 15. A method of bonding a superconductor device with a wire for a high-frequency application comprising bonding a superconductor device with the fine wire according to claim
 1. 16. A method for connecting a semiconductor device with a wire in a flip-chip technique, comprising connecting a semiconductor device with the fine wire according to claim
 1. 